linux-kernel - Re: [PATCH 2/4] samples/bpf: Use llc in PATH, rather than a hardcoded value

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Date:	Thu, 31 Mar 2016 20:19:24 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Alexei Starovoitov <ast@...com>,
	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
CC:	"David S . Miller" <davem@...emloft.net>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>, netdev@...r.kernel.org
Subject: Re: [PATCH 2/4] samples/bpf: Use llc in PATH, rather than a hardcoded
 value

On 03/31/2016 07:46 PM, Alexei Starovoitov wrote:
> On 3/31/16 4:25 AM, Naveen N. Rao wrote:
>> While at it, fix some typos in the comment.
>>
>> Cc: Alexei Starovoitov <ast@...com>
>> Cc: David S. Miller <davem@...emloft.net>
>> Cc: Ananth N Mavinakayanahalli <ananth@...ibm.com>
>> Cc: Michael Ellerman <mpe@...erman.id.au>
>> Signed-off-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
>> ---
>>   samples/bpf/Makefile | 11 ++++-------
>>   1 file changed, 4 insertions(+), 7 deletions(-)
>>
>> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
>> index 502c9fc..88bc5a0 100644
>> --- a/samples/bpf/Makefile
>> +++ b/samples/bpf/Makefile
>> @@ -76,16 +76,13 @@ HOSTLOADLIBES_offwaketime += -lelf
>>   HOSTLOADLIBES_spintest += -lelf
>>   HOSTLOADLIBES_map_perf_test += -lelf -lrt
>>
>> -# point this to your LLVM backend with bpf support
>> -LLC=$(srctree)/tools/bpf/llvm/bld/Debug+Asserts/bin/llc
>> -
>> -# asm/sysreg.h inline assmbly used by it is incompatible with llvm.
>> -# But, ehere is not easy way to fix it, so just exclude it since it is
>> +# asm/sysreg.h - inline assembly used by it is incompatible with llvm.
>> +# But, there is no easy way to fix it, so just exclude it since it is
>>   # useless for BPF samples.
>>   $(obj)/%.o: $(src)/%.c
>>       clang $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) \
>>           -D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
>> -        -O2 -emit-llvm -c $< -o -| $(LLC) -march=bpf -filetype=obj -o $@
>> +        -O2 -emit-llvm -c $< -o -| llc -march=bpf -filetype=obj -o $@
>>       clang $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) \
>>           -D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
>> -        -O2 -emit-llvm -c $< -o -| $(LLC) -march=bpf -filetype=asm -o $@.s
>> +        -O2 -emit-llvm -c $< -o -| llc -march=bpf -filetype=asm -o $@.s
>
> that was a workaround when clang/llvm didn't have bpf support.
> Now clang 3.7 and 3.8 have bpf built-in, so make sense to remove
> manual calls to llc completely.
> Just use 'clang -target bpf -O2 -D... -c $< -o $@'

+1, the clang part in that Makefile should also more correctly be called
with '-target bpf' as it turns out (despite llc with '-march=bpf' ...).
Better to use clang directly as suggested by Alexei.